1. Field of the Invention
The invention relates to the control of brushless motors.
2. Discussion of the Related Art
FIG. 1 shows a cross-section of a typical brushless, DC motor. The motor includes a permanent magnet rotor 12 and a stator 14 having a number of windings (A, B, C shown in FIG. 2). The windings are each formed in a plurality of slots 18. The motor illustrated has the rotor 12 housed within the stator 14. The stator 14 may also be housed within the rotor 12. The invention applies indifferently to either configuration. The rotor 12 is permanently magnetized, and turns to align its own magnetic flux with one generated by the windings.
FIG. 2 shows an electrical schematic diagram of the stator of such an electric motor, and the supply control circuitry used. Often, such motors comprise three phases A, B, C. These may be connected in a star (`wye`) configuration having a common node N (as in the figure), or in a delta configuration. The invention applies indifferently to either. For each winding, a pair of switches XSA, XGA; XSB, XGB; XSC, XGC connect the free end of the winding to supply Vs and ground GND voltages, respectively. The switches are typically power transistors. A reverse biased diode DSA, DGA; DSB, DGB; DSC, DGC is placed in parallel with each of these switches. These diodes are high power rectifiers, and serve to protect the windings against induced voltages exceeding the supply or ground voltage. The opening and closing of the switches may be controlled by a microcontroller. For motors driven by mains power, the supply voltage Vs may be +300V respective to ground GND.
As shown in FIG. 3, the switches are controlled through a sequence of steps. The diagram shows the voltage VA, VB, VC applied to each winding, relative to the common node N. Illustrated in the figure is the case of a motor having phases A, B, C, and controlled through six steps s1, s2, s3, s4, s5, s6 each corresponding to a particular magnetic flux pattern established in the motor. In each of these 6 steps, one of the phases A, B, C is off, and the other two are oppositely polarized. This sequence of steps is a `bipolar` sequence, as the coils may be polarized both positively and negatively with respect to the common node N.
Referring again to FIG. 2, this means that one of the supply switches XSA, XSB, XSC and a non-corresponding one of ground switches XGA, XGB, XGC is closed during each of the steps. One of the windings will be unconnected (although still protected from overvoltages by the protection diodes.) The rotor will align its magnetic flux with that of the stator, and so will rotate synchronously with the stator flux, as it rotates due to the switching of the 6 steps.
If the rotor is already turning, its rotation will induce a back emf voltage in each of the windings of the motor. For a loaded motor, the back emf generated in a winding is approximately in phase with the current flowing in the same winding.
FIG. 4 shows the 6 steps in the voltage VA applied across winding A, the current IA through winding A, and the back emf BemfA generated in winding A by the rotor under load. The high inductance of the windings slows the switching on and off of current IA. The periods of increasing current and decreasing current are the energizing period (pe) and the de-energizing period (pd) respectively. Extending between the end td of each de-energizing period and the subsequent energizing period pe is a surveillance period pz, during which the back emf may be monitored. The back emf alternates in polarity, and passes through zero crossing points zc.
For mains powered motors, such as are commonly used to power domestic appliances, the bipolar DC power supply Vs is directly derived from the AC mains. The DC supply may thus have a value of around +300V relative to the ground voltage for a 230V mains AC supply. The voltage actually supplied to each of the windings of the motor is controlled by pulse width modulation (PWM) of the DC supply voltage. The frequency of the pulse width modulation signal is usually high compared to the rotational frequency of the motor, such as about 10 kHz. This PWM periodically applies then disconnects the supply and ground voltages to the windings. The switches are controlled by a microcontroller as a function of the current needed to be supplied to the motor, both to ensure the switching between steps s1-s6, and PWM control.
During "off" periods between PWM "on" pulses, the motor is freewheeling; the kinetic energy of the motor as it turns is transformed into electrical energy by its rotation in a magnetic field. The motor does not slow down during these periods, as the high PWM frequency and the inertia of the motor and its load makes this change undetectable.
In order for the motor to function correctly, the flux existing in the stator must always be slightly in advance of the rotor, to continually pull the rotor forward. Also, when the flux in the stator is just behind the rotor it is advantageously of a polarity to repel the rotor, to aid rotation. However, the rotor movement and the flux rotation should never be allowed to get out of synchronization, as the rotor may stop turning, or in any case will become very inefficient. Therefore, to optimize the efficiency of the motor, the switching of the windings from one step to another must be controlled in accordance with the actual position of the rotor.
Solutions exist whereby a "self-commutating" mode of operation of the motor is used. This uses monitoring of the back emf generated in the windings, and more particularly, the zero crossing points zc of such back emf, to determine the position of the rotor at a particular time.
A permanent magnet brushless electric motor is classically started up by performing a sequence of steps s1-s6 onto the windings at appropriate step times. The motor operates synchronously, and may be accelerated by increasing the step frequency. When zero crossing events have been detected, the motor is turned to self-commutated mode.
U.S. Pat. No. 4,654,566 describes such a motor control system. The zero crossing information is then used to switch to the next step in the sequence.
The solution offered by this patent uses only an approximation of the back emf voltage, which is thus inherently inaccurate. A simulated common node N voltage is used, not the actual common node voltage. Circuitry required to perform this approximation requires a significant number of high accuracy resistors and capacitors, which are expensive. These components may need to have an accuracy better than .+-.1%. The PWM used to supply the motor induces a large amount of electrical noise, which must be filtered out. In order to work at many speeds, the time constant of the filter needs to be variable. This requires switching arrangements and more high accuracy components to effect the switching. The use of integrating filters introduces a significant delay in reacting to changes in the situation of the motor. Measurement of back emf suffers from noise due to the PWM, and other noise. The back emf is scaled by a divider, which results in a back emf signal that is very small. The signal/noise ratio is thus extremely poor.
During the de-energizing period pd, it is not possible to measure back emf, as a large current is flowing in the `unconnected` winding. Therefore, the back emf measurement must be disabled during this period. The length of the de-energizing period is not constant, but depends on the motor, its speed and loading, and on any dissymmetry between the phases of the motor. Known solutions (U.S. Pat. Nos. 4,654,566; 5,172,036) either assume a fixed de-energizing period, or `anticipate` the next zero crossing point from the timing of the previous zero crossing point. To be able to work under all conditions, the fixed de-energizing period must be longer than the longest possible actual de-energizing period. This reduces the length of the surveillance period td that back emf can be detected in, and so reduces the operational range of the circuit. Such a fixed period leads to a fixed maximum speed of rotation of the motor. The other known `anticipate` solution also has drawbacks. Dissymmetries between the phases of the motor result in a time interval between zero crossing points which is not fixed. Therefore, the dissymmetries render the `anticipation` of the zero crossing points ineffective.